This invention relates to a wet surface air cooling system of the evaporative type in which the cooling effect is obtained primarily from the evaporation of water directed onto the exterior of a bundle of exchanger tubes arranged in an airstream passing over the tubes. More particularly, the invention relates to a system of increasing the capacity of a wet surface air cooling unit of the character described.
Air cooling systems of the character described are designed to achieve the required process fluid cooling and/or condensing when a preselected maximum summer design ambient air wet bulb temperature exists. At any temperature lower than the preselected maximum ambient wet bulb temperature, the systems are capable of achieving a higher than designed process fluid cooling or condensing rate. However, the design wet bulb temperature is achieved only from about 1 to 5 percent of summer hours. Consequently, most cooling systems are over-designed to accommodate a relative small percent of their usage time. This, of course, increases the cost of a unit which is operating under capacity the majority of its operating hours.
This invention is directed to a system which employs a secondary cooling unit which is considerably less expensive than conventional exchanger tube units to accommodate those relatively few conditions where full load capacity is required. More particularly, in cooling units which employ a bundle of heat exchanger tubes, a collection basin or sump collects water directed over the exchanger tubes. When a wet surface cooling system is running at full design condition, i.e. at full capacity, the water in the collection basin or sump will reach a particular temperature. When the collected water has reached the maximum temperature and is recycled and sprayed over the heat exchanger, the cooling unit becomes inefficient. This invention contemplates sensing a particular temperature of the water collected in the collection basin, or by other sensing systems, and providing means for decreasing the spray water temperature to increase the capacity of the cooling unit. This is accomplished by employing a secondary cooling unit, such as a cooling tower, refrigeration, or other cooling means, and employing circulation means for drawing a portion of the water from the collection basin, cooling the water through the secondary cooling tower, returning the cooled water back to the collection basin and thereby decreasing the temperature of the water in the basin which is directed over the bundle of heat exchanger tubes. The secondary cooling unit or tower, therefore, only needs to be operative when the unit is subjected to full cooling load and with high ambient wet bulb temperature.
In the exemplary embodiment of the invention, a wet surface air cooling system is provided and comprises a primary cooling unit which includes heat exchanger means, means for directing water over the heat exchanger means, and a collection basin for collecting the water directed over the heat exchanger means. A secondary cooling unit includes a cooling tower and circulation means for selectively drawing a portion of the water from the collection basin and directing the water to the cooling tower to increase the cooling capacity of the system. Return means is provided for returning the portion of the water back to the collection basin.
As disclosed herein, the circulation means, the cooling means whether by a cooling tower or other cooling means, and the return means retain the advantages of having closed loop process fluid cooling whereby the heat exchanger tube bundle contents are closed to the introduction of contaminants. The circulation means includes pump means and selectively operable valve means. The heat exchanger means includes a bundle of heat exchanger tubes and the means for directing water over the heat exchanger means comprises a spray head.
The advantages of the increased capacity wet surface air cooling system of the invention are considerable. The original design size or capacity of conventional wet surface units, such as those employing expensive heat exchanger tubes, is drastically reduced. The secondary cooling unit can be used to retroactively increase the capacity of an existing wet surface unit without simply adding more, expensive heat exchanger tube bundles. The secondary cooling unit can be located at a distance from the primary wet surface cooling unit. Thus, the secondary cooling unit can be incorporated as part of the design of a cooling system to have a predetermined capacity or retroactively, as described. The system is particularly applicable for industrial applications, such as air conditioning installations.